The present invention relates generally to a lock-up type automatic transmission, and particularly to an improvement in a lock-up control whereby the occurrence of substantial shift shocks is prevented.
Commonly, automatic transmissions have a torque converter in order to multiply the torque from an engine. In the torque converter, an inlet element (a pump impeller) driven by the engine imparts a rotational force to a hydraulic fluid contained in the torque converter, and the rotation of the fluid causes an output element (a turbine runner) to rotate under the reaction of a stator, thus multiplying the engine torque (this mode of operation being called the "converter state"). Owing to this hydrodynamic transmission of power, the torque converter is subjected to a slip between the pump impeller and the turbine runner while it is in operation. Thus, automatic transmissions having a torque converter exhibit poor fuel economy due to poor power transmission efficiency although they are easy to operate. In order to alleviate this drawback, there has been proposed a torque converter with a so-called lock-up or bridge clutch (which may be called as a lock-up torque converter) wherein the turbine runner is directly and mechanically connected to the pump impeller (this mode of operation being called as "a lock-up state") in order to eliminate the slip at relatively high vehicle speed range where the engine is not subject to substantially torque variations. Recently, some automobiles have begun to use such an automatic transmission with a lock-up torque converter.
Referring to FIG. 7, a shift pattern diagram illustrates lock-up ranges of an automatic transmission wherein the torque converter with the lock-up clutch assumes the lock-up state when an actual vehicle speed is higher than a preset vehicle speed value (i.e., a lock-up vehicle speed) for each of forward speed or gears. In this Figure, there is illustrated a shift schedule for upshiftings to take place in a three-speed automatic transmission, where V1, V2 and V3 designate lock-up vehicle speeds for the first, second and third speeds, respectively, and A, B and C designate lock-up ranges for the first, second and third speeds, respectively. In the case of automatic transmission where the torque converter locks up when the vehicle speed is higher than the lock-up vehicle speed as mentioned above, a shifting between two adjacent forward speeds with the accelerator pedal depressed deeply by great degree (with a large throttle opening degree) takes place when the torque converter remains in the lock-up state thereof as will be readily understood from the fact that the lock-up ranges A, B and C are disposed one next to another along the vehicle speed at the large throttle opening degrees. If shifting were to take place with the torque converter in the lock-up state, substantially great shocks would take place because the torque variations upon shifting could not be absorbed.
In the lock-up type automatic transmission of this kind, the above mentioned problem has been partly solved although not completely by releasing the lock-up action upon shifting even during operation in any one of the above mentioned lock-up ranges, thereby temporarily causing the torque converter to operate in the torque converter state. It is therefore the common practice to provide a shift detecting circuit which generates a shift indicative signal for a predetermined period of time after a command for shifting has been made and the lock-up action is interrupted temporarily while the shift signal from this circuit is present even during operation in any one of the lock-up ranges.
Describing how the above mentioned shift detecting circuit works in shifting from the second speed to the third speed referring to FIG. 10A, it generates a shift signal for a predetermined period of time T' at the same instant t1 when a command for shifting takes place for releasing the lock-up (L/u) action. According to this control strategy, the lock-up action is released too early, thus posing problems as follows. There is a delay from the instant t1 when the command for shifting is made to the instant t3 when the actual shifting operation initiates, viz., initiation of actuation of the friction elements, because of a response delay in the hydraulic control system of the transmission. Therefore, if the lock-up action is released simultaneously with the instant t1 when the command for shifting is made, the lock-up action is released before the actual shifting operation begins, so that engine revolution speed rises rapidly during a moment from t1 to t2 as shown in FIG. 10A, causing the engine to race. Owing to the fact that releasing the lock-up action terminates so as to allow the lock-up action to resume during a moment from t3 to t4 when the shifting operation actually takes place, the torque converter cannot aborb shift shocks inherent with the shifting operation, and this fact is combined with the fact that engine revolution speed rises resulting from engine racing as mentioned, causing the magnitude of peak torque to occur at the instant t5 immediately after the shifting operation, with the inevitable result of substantial shift shocks. This tendency becomes marked when the automatic transmission is subject to an upshifting because the upshifting takes place with the power-on mode. This problem is more serious upon upshifting from the second speed to the third speed where a front clutch that is to be engaged for the third speed is engaged while releasing a second brake which is to be applied for the second speed because the completion of the actual shifting operation is delayed further due to a long overlap time in shifting.
As one measure to solve this problem, it is conceivable to elongate the lock-up interrupt time T' up to the instant when the shifting operation is to be completed, but this leaves the racing problem of the engine unsolved. The increase in the engine revolution speed causes the corresponding increase in the magnitude of shift shocks.
For solving the above mentioned problem, U.S. Pat. No. 4,431,095 issued to Massaki Suga on Feb. 14, l984 has disclosed a lock-up type automatic transmission wherein a delay circuit is provided so as to delay generation of a shift signal for a predetermined time T1 after the instant t1 when the command for shifting is made as shown in FIG. 10B, whereby the interruption (OFF) of the lock-up (L/u) action, where is to take place upon shifting during operaion in any one of the lock-up ranges, begins at the instant t3 when the actual shifting operation begins.
However, the delay time from the instant t1 to the instant t3 (i.e., the delay from the instant when the command for shifting is made to the instant when the actual shifting operation initiates) varies from one manufacturing product to another due to manufacturing dispersion among products, i.e., a difference in flow resistance in shift control fluid passages and variation in viscosity of hydraulic fluid, and it has been confirmed that the predetermined time T1 set by the above mentioned delay circuit does not necessarily agree with the delay time from t1 to t3 with the result that with this conventional measure the above mentioned engine racing and substantial shift shocks cannot be prevented.
The period of time from t3 to t4 taken for actual shifting operation is subject to variation due to the manufacturing dispersion among products although the amount of such variation is not as large as the magnitude of variation experienced in the delay time from t1 to t3. Thus, it cannot be said that the lock-up interrupt time T' is always equal to the actual time from t3 to t4. Therefore, even if the predetermined time T1 agrees well with the delay time from t1 to t3, the lock-up interrupt releasing timing does not always agree with the termination of the actual shifting (the instant t4), thus failing to accomplish complete prevention of shift shocks.
Considering the variation in the transmission output torque during actual shift operation and before and after the duration, as shown in FIGS. 10A and 10B, the transmission output torque starts to drop at the instant t3 when the actual shift operation is initiated because a first friction element which has been engaged is released (i.e., a second brake in the case of 2-3 upshift). Thereafter, the transmission output torque increases again because of the engagement of a second friction element scheduled to be engaged subsequently (i.e., a front clutch in the case of 2-3 upshift) and continues to increase after the instant t4 when the engagement of the second friction element is completed because of the inertia of the engine. This increasing torque reaches a peak at the instant t5. After experiencing this peak, the torque decreases to a level which the transmission output torque should assume after the shift operation and reaches this level after the instant t6. In other words, the time derivative of the transmission output torque becomes very small after the instant t6.
The occurrence of shift shocks is brought about by the torque variation which begins with the instant when the transmission output torque exceeds an extending line of the torque variation trend occurring immediately before the instant t3 after the actual shifting operation has begun and which ends with the instant t6 when the transmission output torque reaches the certain level. Therefore, if the lock-up is interrupted during this period of time, the shift shocks are suppressed to a sufficiently low level.
Accordingly, an object of the present invention is to provide a lock-up control which causes a shockless gear shifting by precision control of a lock-up interruption timing and duration.